Partial bandgap characteristics of parallelogram lattice photonic crystals are proposed to suppress the radiation modes in a compact dielectric waveguide taper so as to obtain high transmittance in a large wavelength range. Band structure of the photonic crystals shows that there exists a partial bandgap, The photonie crystals with partial bandgap are then used as the cladding of a waveguide taper to reduce the radiation loss efficiently. In comparison with the conventional dielectric taper and the complete bandgap photonic crystal taper, the partial bandgap photonic crystal taper has a high transmittance of above 85% with a wide band of 170 nm.
A two-dimensional （2D） optimized nanotaper mode converter is presented and analyzed using the finite- difference time-domain （FDTD） method. It can convert the mode size in a silicon pillar waveguide （PWG） from 4 μm to 1 μm over a length of 7 μm and achieve a transmission efficiency of 83.6% at a wavelength of 1.55 μm. The dual directional mode conversion of the nanotaper and its ability to perform mode compression and expansion are also demonstrated. The broadband with high transmittance is satisfied in this structure. Using this silicon-based nanotaper, mode conversion between integrated photonic devices can be more compact and efficient.
We discuss the optimal design of line-tapered multimode interference （MMI） devices using a genetic algorithm （GA）. A 1×4 MMI device is designed as a numerical example. Compared with the conventional design based on self-imaging theory, the present method demonstrates superior performance with low in- sertion loss and small non-uniformity.
